Exhaust gas sensors are used in a variety of applications that require qualitative and quantitative analysis of gases. For example, exhaust sensors have been used for many years in automobiles to sense the presence of selected exhaust gases. In automotive applications, the direct relationship between various exhaust gas concentrations and the air-to-fuel ratios of the fuel mixture supplied to the engine allows the sensor or sensors to provide concentration measurements for determination of optimum combustion conditions, maximization of fuel economy, and management of exhaust emissions.
For example, U.S. Pat. No. 6,616,820 to Wang et al. describes in the Abstract a gas sensor for sensing NOx (nitrogen oxides) having electrochemical cells wherein dielectric material surrounds electrolytes except where electrodes are attached. With the use of this technique, signal cross talk is minimized while enhancing NOx sensing sensitivity. Further, the total number of electrodes needed are reduced which allows for more complex sensor structures.
U.S. Pat. No. 6,797,138 to Detwiler et al. describes in the Abstract a gas sensor comprising a first electrode and a reference electrode with an electrolyte disposed therebetween, wherein the first electrode and the reference electrode are in ionic communication, wherein the reference electrode has a surface on a side of the reference electrode opposite the electrolyte and the surface has a surface area. The gas sensor also comprises a reference gas channel in fluid communication with the reference electrode, wherein at least a portion of the surface of the reference electrode physically contacts at least a portion of the reference gas channel, and wherein the portion of the reference electrode in physical contact with the reference gas channel is less than about 90% of the surface area.
U.S. Pat. No. 6,579,435 to Wang et al. describes in the Abstract a gas sensor and a method of using a gas sensor. The gas sensor comprises an oxygen pump cell having at least one exterior pump electrode and at least one interior pump electrode disposed on opposite sides of a first solid electrolyte layer. An emf cell having first and second emf electrodes and first and second reference gas electrodes are disposed on opposite side of a second solid electrolyte layer. At least one insulating layer is in contact with the first and second emf electrodes. At least one via hole is disposed through the first solid electrolyte layer. At least one air channel is disposed through at least one insulating layer. An air vent is disposed in at least one insulating layer in contact with the first and second reference gas electrodes. A heater is disposed in thermal communication with the sensor. And at least five electrical leads are in electrical communication with the sensor.
Particular to NOx sensors, treatment of the exhaust gas is employed prior to being analyzed using, for example, the Nernst and/or polarographic principles. Typically, this is achieved using catalyst and/or by maintaining the other gases at constant levels within an enclosed or semi-enclosed environment. Once the exhaust gas is treated, the gas encounters the sensor's electrochemical cells.
In order to meet some emission regulations, selective catalytic reduction systems using externally added reducing agents may be used. In such systems, regulated emissions, such as certain nitrogen oxides, or NOx, can be reduced in an oxygen-rich environment to nitrogen and water over a catalyst when a reducing agent, such as ammonia, is added. In addition to controlling nitrogen oxide emissions, the amount of excess ammonia, or ammonia slip, must be managed. Ammonia slip is experienced when ammonia in excess of that used to reduce the nitrogen oxides passes through the catalyst unaffected and exits the catalyst (as ammonia slip).
One method for regulating ammonia slip is to use an ammonia sensor located downstream of the catalyst. The detected ammonia concentration is compared with a fixed upper threshold value. This comparison generates a correction signal that is used to control the metering of ammonia upstream of the catalyst. In this scheme, it is believed that by regulating actual ammonia slip to the upper threshold value, a certain nitrogen oxide reduction is obtained. Such a system is disclosed in U.S. Pat. No. 5,369,956. Reference also U.S. Pat. Nos. 6,295,809 and 6,532,736.
The disclosures of each of the foregoing U.S. Patents are each incorporated herein by reference in their entireties. The appropriate components and process aspects of the each of the foregoing U.S. Patents may be selected for the present disclosure in embodiments thereof.
There remains a need for an improved exhaust gas species sensor and an improved method for preparing such a sensor. There further remains a need for a durable and fast response ammonia sensor and a method for preparing the same. Particularly, there is a need for an ammonia sensor that is stable at high temperature and high humidity environments, such as diesel engine exhaust applications, which is able to withstand exposure to diesel exhaust impurities (e.g., soot) with little or no degradation.